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Questions and Answers
Which material does this text specifically focus on in terms of corrosion?
What is the most general case of corrosion?
What does the term 'rusting' specifically refer to?
What does 'rust' refer to in the context of corrosion?
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In which color does rust typically appear?
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Which of the following materials do not rust but still corrode according to the text?
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What are common units for the corrosion current density?
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Which unit is used for expressing the corrosion rate as a uniform penetration rate?
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What unit is equivalent to mpy (mils per year)?
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What is considered an unacceptable corrosion rate in terms of corrosion current density?
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What are some units that have not held wide appeal to corrosion scientists or engineers?
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How can the corrosion rate in one unit be converted to another unit?
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What is the term often used to describe the powdery white corrosion product formed on zinc?
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Which physical process involves the rubbing or sliding of materials on each other?
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What is the term used to describe the failure of a metal under an applied repeated cyclic stress?
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What can lead to 'white rusting' on sheets of galvanized steel?
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Which process involves the mechanical damage caused by the movement of a liquid against a metal surface?
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In what environment can physical degradation processes be assisted or aggravated?
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What happens to the metal lattice when a metal is cold worked or shaped?
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In which type of solutions do acid solutions have pH values?
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What is an electrolyte?
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What is the role of the metallic path in corrosion?
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What does the pH measure in a solution?
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What happens to the positions of anodic and cathodic surface sites with time during corrosion?
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What is the product of [Al3+][OH−]3 when [Al3+] is 1.0 × 10−6 and [OH−] is 1.0 × 10−5?
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Why does Al(OH)3 precipitate under the given conditions?
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What is the unit of charge used in Faraday's Law?
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How is mass transfer related to charge transfer in corrosion?
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What does Faraday's Law relate to the mass of metal corroded?
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What is the relationship between the Faraday and the number of equivalents transferred per mole of metal?
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Study Notes
Units for Corrosion Rates
- Corrosion current density units: microamperes per square centimeter, milliamperes per square centimeter, and amperes per square meter
- Mass loss units: grams per square centimeter per day, mdd (milligrams per square decimeter per day)
- Uniform penetration rate units: ipy (inches per year), inches per month, mpy (mils per year), millimeters per year, and micrometers per year
- Corrosion rates > 100 μA/cm² are considered unacceptable
What is Corrosion?
- Corrosion: destructive attack of a metal by its reaction with the environment
- Refers specifically to metal degradation, while other materials undergo deterioration
- "Rusting" applies to corrosion of iron and plain carbon steel, while non-ferrous metals corrode but do not rust
Physical Processes of Degradation
- Fracture: failure of a metal under an applied stress
- Fatigue: failure of a metal under an applied repeated cyclic stress
- Wear: rubbing or sliding of materials on each other
- Erosion or cavitation erosion: mechanical damage caused by liquid or vapor bubble collapse against a metal surface
- Radiation damage: interaction of elementary particles with a solid metal to distort the metal lattice
Environmentally Assisted Degradation Processes
- Physical degradation processes can be assisted or aggravated in the presence of an aqueous environment
- Cold worked or shaped metals can lead to uniform corrosion due to strain-induced dissolution
Requirements for Corrosion
- Four conditions necessary for corrosion to occur: • An anodic reaction • A cathodic reaction • A metallic path of contact between anodic and cathodic sites • The presence of an electrolyte (e.g., aqueous solution)
Three Simple Properties of Solutions
- pH: a measure of acidity (or alkalinity) of a solution
- pH = −log[H+] (where [H+] is the concentration of hydrogen ions in solution)
- Neutral solutions have a pH value of 7.0, acid solutions have pH values < 7.0, and alkaline solutions have pH values > 7.0
The Faraday and Faraday's Law
- The Faraday (F): 96,500 C/equivalent
- Faraday's law states that the mass (w) of metal corroded is given by: W = (I * t) / (n * A * F)
- Where I is the current in amperes, t is the time in seconds, A is the atomic weight of the metal, and n is the number of equivalents transferred per mole of metal
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Description
Learn about white rust, the powdery white corrosion product on zinc-coated steel, and red rust, the corrosion product formed on steel. Understand how condensation of moisture between stacked sheets can lead to white rusting issues. See examples of red rust corrosion near the waterline of ships.
